Hi.
When playing the game on xbox 360 during the quest "You'll Know It When It Happens" on "Yes man's" side.
After saving the president from assassination the quest "Eureka!" isent given an making the "NCR" quest line impossible to complete.

Going to "Colonel Cassandra Moore" without the quest she just says: "whatever it is make it quick"
And there is just one speech option were you reply along the line "Nothing just siteseeing"
And there is no other way possible to acctivate the quest "Eureka!"
Then been given it after savning the president.

Any ideas how to resolve the problem without restarting from the begining of the game?

No, but I suggest you don't post spoilers in your titles!!!

Bump

Did you wait outside after saving the president until it said you completed the quest? This takes about a good minute or two after saving the president. Then Moore will ask the status of the president. The conversation in this automatically continues and will mention so and so wants to see you, and then you are given the option to continue on or return later. Selecting to continue will trigger Eureka, and is a one way trip to the end of the game.

You could also check your completed quest log to see if Eureka is in there as a failed quest due to some other actions you took earlier in the game.

Moore never comes up to me and starts a convertation, and i dont have Eureka! as a failed quest.
All Moore says is the same bullcrap "whatever it is make it quick"

don't know what to tell ya, the order of events for me was pretty quick though.
Then again, I did this from the NCR path to begin with.

-Save president
-Wait outside until mission is shown as completed
-Enter hoover dam and talk to moore
-Select to accept eureka! or return later.

Only things that come to mind are maybe a lack of NCR fame or not waiting outside until the quest completed. I believe my NCR fame was IDOLIZED.
Seeing as how Eureka is the endgame NCR side, if you didn't do things like For The Republic (Part 2), and whatnot, I believe you have to do those first.
There are various side quests that tie in with the NCR direction, some of which you can get doing other quests, but does not link into the NCR plot until you reach it directly doing NCR quests.
I had a hell of a time getting to Eureka!...

I have tryed to bring my fame/reputation up but i have done all the NCR missons before "You'll Know It When It Happens".
So i tryed to just go out and kill lots of Caesar's Legion troops but i dident get any NRC fame.